Another slant on the question:
In Australia, we have situations where the rostered controller can't attend work, or we are short staffed so that a shift is for some reason vacant. We don't have staff on call, or spare staff on shift, so at short notice the few controllers on days off are called at home. No one with the appropriate endorsements is available to come in. So for a period of time, the ATC service in that sector isn't available = TIBA.
While there is no qualified ATC available, the TIBA procedures are applied. The frequencies associated with the airspace are not monitored (or may be monitored for emergency transmissions by a Manager not endorsed on the airspace). No separation is provided.
What happens in other countries? How do you manage short notice absences? Clearly no one has the same problems that we do here.